For many years electrostatic principles have been employed in the generation and reception of acoustic energy. Devices based on electrostatics include the electrostatic loudspeaker and the condenser microphone. Such devices consist of three basic elements: (1) a rigid, electrically conducting backplate; (2) a flexible, electrically conducting diaphragm; (3) one or more mechanically compliant dielectric layers. These three elements are arranged in sandwich form with the dielectric in the middle. An electric bias voltage applied across the two conducting elements generates an electrostatic force which is balanced by the mechanical restoring force of the dielectric, establishing an equilibrium separation between the diaphragm and the backplate. For acoustic generation, the bias voltage has superimposed on it a time-varying voltage causing the diaphragm to vibrate. For acoustic reception, the time-varying voltage across the diaphargm and backplate is proportional to the vibration of the diaphragm in response to an incident acoustic field.
In order to reduce or eliminate the required dc bias voltage (which may be several hundred volts), an electrostatic device may be built with an electret as a dielectric element. Electrets, which are polarized dielectrics, can be formed in various ways, one of which will be described later. Using an electret in a condenser microphone results in an electret microphone in which the electret, rather than an external voltage source, generates the necessary electrostatic force.
Electrostatic devices, with or without electrets, have been widely used for audio applications. In such cases, the acoustic wavelength is larger than the lateral dimensions of the devices. Thus it can be assumed that the diaphragm vibrates uniformly as a sound piston.
For acoustic imaging purposes, energy in the ultrasonic region is utilized. The acoustic wavelength is usually much smaller than the detector lateral dimension. When a target is illuminated with an ultrasonic beam, the target attenuates part and scatters the rest of the beam. Hence the sound reflected from or transmitted through the target is, in general, no longer of uniform amplitude and phase. Instead, the beam is modulated, in both amplitude and phase, on a point-by point basis.
It is a well-known result of diffraction theory that if the point-by-point amplitude and phase of a field scattered by a target can be detected on some plane, and if this amplitude and phase distribution can be recorded and reproduced, an image of the target can be reconstructed. Further, if the incident field is acoustically derived, while the playback is optical, then an optical reproduction of the acoustically-illuminated target is generated. This is the basis of acoustical holography.